Decarburization in steels

1. The problem statement, all variables and given/known data
I understand that decarburization happens during heat treatment of steels @ 700 C, and that carbon reacts with oxygen or hydrogen that is in the atmosphere. But, where does the carbon go?

2. Relevant equations
None3. The attempt at a solution
Does the carbon react with the oxygen and turn into Carbon Oxide like on the surface of the steel? Or does it turn into CO2?